The present invention relates to an induction melting furnace comprising a crucible having an inductor surrounding the crucible and containing a metal or metal alloy, with segments and slots in the crucible.
When melting substances in a crucible, care must be exercised to insure that the melting temperature of the crucible is higher than the melting temperature of the material to be melted. This difference in melting temperature must be maintained, since in traditional crucibles, the inside surface of the crucible has the same temperature as the melt. A ceramic crucible will usually meet this requirement since it has an extremely high melting temperature.
A ceramic crucible's highly heated inside surface, however, can chemically react with the melt. The crucible material will then contaminate the melt. The contamination usually takes the form of oxidation of the melt upon reduction of the crucible oxide ceramic. It is also possible for the crucible's impurities, for example sulphur, to enter into solution. In addition, flakes of ceramic particles can come off the crucible and fall into the molten material. After solidification of the molten material, the flakes can form inclusions. These inclusions are often referred to as "low density inclusions" and diminish the quality of the solidified melt since, for example, the inclusions are the starting points for cracks.
One possibility of avoiding these problems is to manufacture the crucible of metal instead of ceramic. With a metal crucible, however, a substance having a high melting temperature cannot be melted in the crucible. For example, if a copper crucible, without any special measures, were used to melt a metal such as tantalum, tungsten, or thorium, the copper crucible would melt long before the melting temperature of these metals was reached.
In order to melt substances with a high melting temperature in a crucible made of a metal with a lower melting temperature, it has long been known to cool the crucible with water. As a result, the crucible is continuously held at a temperature below its melting point The problem with this device, however, is in heating the material to be melted. Since the container holding the material is itself cooled, the container cannot permit any material to be melted to reach any higher temperatures.
The problem can be resolved in a simple way by electrically heating the material to be melted, namely by induction heating. A coil, supplying electrical energy to the material to be melted, through the crucible wall, is thereby provided around the crucible The metal crucible can then be composed of individual segments, separated from one another by an insulating layer so that the crucible is not excessively heated by eddy currents from the induction heating (see German Pat. No. 518 499). The insulation could be, for example, mica.
Another known induction melting furnace for melting metals comprises an oblong, electrically insulated, and water-cooled melting crucible that is open at the top and the bottom and has the same cross-section over its entire length (U.S. Pat. No. 3,775,091). This melting crucible is divided, by vertical slots, into at least two segments. Every segment is electrically insulated from the other segment so that no electrical shorts occur. The slots serve the purpose of reducing the shielding action of the crucible for the electric fields. The furnace has a ceramic lining in order to always produce and maintain insulation between the segments and at the inside of the crucible. The ceramic lining has electrical insulating properties in its solid state and a melting point temperature that differs from the melting point temperature of the metal to be melted. The ceramic insulating lining, for example, contains an alkaline earth metal fluoride. A self-generating insulating lining is produced as a result thereof.
A disadvantage of this furnace is that the employment of slag, when melting reactive metals, has the risk of contaminating the metal. It has also been shown that the quality of the molten material leaves a great deal to be desired, even given a partial pressure of argon or helium.
The employment of insulating slag between the melt and the cooled segments is not necessary for electrical reasons, as disclosed in German Patent 518 499. The insulating layers between the melt and the cooled segments are utilized since they represent a heat insulating layer and, thus, noticeably reduce the heat flux from the melt to the cooled segments. Therefore, the melting can be performed with a lower electrical induction heating capacity. The size of the power supply can then be lowered, and the current forces that limit the process are not yet that noticeable.
A known method for induction melting of reactive metals and alloys in a non-reactive environment comprises melting the material to be melted in a crucible subdivided into segments, without insulating slags (EP-A-0276544). This method is intended to produce qualitatively, high-grade products that have not been contaminated by slags or the like. Therefore, the wall segments of the crucible are not electrically insulated from one another but are connected to one another at their base and thus electrically shorted, as disclosed in the preceding publications and in German Patent 518 499. In addition, the crucible is provided in an evacuated space having less than 500 .mu.m Hg. The disadvantage of this method is that the induction introduced electric heating capacity is the same over the entire height of the crucible and does not lead to an optimum melting time.
In diffusion blast furnaces, vacuum furnaces, and pottery kilns, the heating region in each is subdivided into various zones. A different coil can then be used for each zone (U.S. Pat. No. 3,291,969; German Published Application 21 52 489; U.S. Pat. No. 4,011,430; and German Pat. No. 27 04 661). These furnaces, however, are not suitable for melting materials which have a melting temperature which is higher than the melting temperature of the crucible.
A fundamental disadvantage of the above-described melting processes, which have a cooled crucible, is that the material to be melted suffers high losses of energy by emitting heat to the crucible wall. The thermic process efficiency can only be kept at an acceptable level by performing the melting process as quickly as possible. Therefore, the quantity of energy dissipated as heat loss, as a product of dissipate power and time, is small.